1. Field of the Invention
This invention relates generally to power supply systems and more particularly to a system and method for providing double fault protection within a digital camera device.
2. Description of the Background Art
Product safety and reliability are important considerations of designers, manufacturers and product users. Electronic power supply devices are particularly important areas for such product safety and reliability considerations. Portable electronic products may receive operating power from an internal battery or an external power supply. When a single portable electronic device (such as a digital camera) alternately uses both an internal battery and an external power supply, the safe operation of the digital camera may merit additional safeguards. Therefore, product designers may wish to include a second backup safeguard device to protect against possible failure to the primary safeguard device. This practice of adding a second duplicate safeguard device is known as "double fault protection."
Referring now to FIG. 1, a schematic of a conventional double fault circuit is shown. Battery 10 provides operating power from positive terminal 12 through diode 16 to the DC input of the device power supply (not shown) via line 18. The negative terminal 20 of battery 10 is connected to line 22 which is connected to switch 26 by line 24. When switch 26 is closed, line 24 is connected to ground 30 via line 28. The battery circuit is thus completed and current flows to supply operating power to the device power supply.
However, if external power is connected to external power connector 32, then battery 10 must be disconnected from the FIG. 1 circuit, so that the external power is not applied directly across the terminals of battery 10. If the external power is applied directly across terminals 12 and 20 of battery 10, then a danger exists that battery 10 could overheat and possibly rupture, thereby endangering the device and device users.
In the FIG. 1 design, an external power source may be connected to external power connector 32 using a conventional barrel plug 50 having an internal positive terminal 52 and an external negative terminal 54. If the plug is inserted, then external power connector 32 provides operating power from positive terminal 36, via line 38, through diode 40, to line 18 which connects to the device power supply. Negative terminal 34 connects to ground 30 via line 28, to complete the circuit and allow operating power to flow to the device power supply.
Inserting the external power plug 50 into external power connector 32 also opens switch 26 to disconnect battery 10 from the FIG. 1 circuit. In addition, diode 16 provides double fault protection by becoming reverse-biased when a higher external voltage is present on line 42, thus effectively disconnecting battery 10 from the circuit. The FIG. 1 design thus requires failures (faults) in both switch 26 and diode 16 to connect the external power source directly across terminals 12 and 20 of battery 10.
In portable electronic devices (such as digital cameras), the conservation of battery power is another important design consideration. In the FIG. 1 circuit, a significant voltage drop is caused by diode 16 (the secondary fault protection device discussed above). This diode 16 voltage drop results in less available power from battery 10. Under heavy load conditions (such as charging an electronic flash in a digital camera), the problem is accentuated. As battery 10 discharges, the diode 16 voltage drop will accelerate a power failure condition in the powered device. Therefore, an improved system and method is needed for providing double fault protection within a digital camera device.